1. Field of the Invention:
This invention relates to a ventilation regulated hot air supplied constant temperature oven, and more particularly to the regulation of ventilation of a hot air supplied constant temperature oven used in a heat aging test for a sample of, for example, rubber and a plastic material.
2. Description of the Prior Art:
In general, in a heat aging test for a sample of rubber and a plastic material, the frequency in carrying out the ventilation of a test oven is controlled suitably, i.e., the number per hour of changing the air in the test oven during a test, the volume of which air corresponds to the capacity of the test oven, is set to not less than one. A difference in the frequency in carrying out the ventilation of the oven causes large variations of the results of the tests.
The conventional methods of measuring the ventilation frequency in such a constant temperature oven include a method of measuring a flow rate in an air discharge cylinder. However, since the durability of a flow meter with respect to temperature is low, the frequency in carrying out the ventilation of the oven is calculated generally by measuring the power consumption of a heater according to ASTM E 145 standards.
This method of calculating the frequency in carrying out the ventilation of the oven consists of the steps of determining an average power consumption, which is required to maintain a test temperature at the same level, of a heater with the air vents of the test oven closed so as to put the interior thereof in a non-ventilating state, determining an average power consumption, which is required to maintain the test temperature at a preset level, of the heater with the air vents of the test oven opened so as to put the interior thereof in a ventilating state, and then calculating the frequency in carrying out the ventilation, which can be used as an index of a flow rate of the air passing through the interior of the test oven while the air vents thereof are opened, on the basis of a difference between the power consumption determined in the two previous steps.
In this method, the temperature in the test oven is set to a level higher than the ambient temperature by 80.degree..+-.2.degree. C., and the temperature is increased so as to obtain thermal equilibrium, the power consumption being measured after the thermal equilibrium has been obtained.
Determining the frequency of ventilation in this manner requires a long period of time, and inaccurate results are obtained due to the variation of the temperature of the outside air, varialtion of power source voltage and variation of wind speed. Moreover, as a test proceeds, the sample in the oven generates a gas due to a chemical reaction caused by the high-temperature air, and the gas thus generated causes the discharge cylinder to be clogged, and the flow rate of the ventilating air to vary.
For example, during an operation for determining the frequency of ventilation, it takes at least 30 minutes to obtain thermal equilibrium after the attainment of a temperature higher than a prescribed ambient temperature by 80.degree..+-.2.degree. C., and not less than 30 minutes each time to determine the power consumption.
The determining of the power consumption is done three times with the test oven in a non-ventilating state, and three times with the test oven in a ventilating state, so that it is necessary to spend a long period of time for carrying out these measurement operations. If the power source voltage varies during the measurement of power consumption, it is difficult to obtain thermal equilibrium in the test oven. Especially, in the case where the frequency of ventilation is as low as not more than 10 times per hour, a difference between the power consumption determined with the test oven in a non-ventilating state and that determined with the test oven in a ventilating state is small. Therefore, more time is required and the calculated frequency of ventilation is inaccurate.
The variation of the ambient temperature causes heat radiated from the test oven to vary and the power consumption to differ. The matutinal, daytime and nighttime temperatures in the test oven during operation differ usually to a great extent. When a temperature difference exceeds a prescribed level, it is necessary to interrupt the test and restart a measurement operation for determining the frequency of ventilation. If the test is continued without carrying out this operation, the calculation of the frequency of ventilation is made on the basis of the level of power comsumption which is determined initially with the test oven in a non-ventilating state in which the air vents thereof are closed. Therefore, the accuracy of the frequency of ventilation thus determined becomes low, and, especially, a lower level of frequency of ventilation makes the accuracy worse.
Even when a sample placed in the oven is tested under the conditions of a preset frequency of ventilation, the gas and plasticizer discharged from the sample into the discharge cylinder of the test oven are gradually condensed and deposited to cause the inner diameter of the cylinder to decrease. As a result, the flow rate of the air being changed which is discharged from the discharge cylinder varies with the lapse of time, and tends to increase the internal pressure of the test oven.
However, a conventional oven of this kind is not provided with a means for detecting an increase of the internal pressure thereof. Consequently, the frequency of ventilation varies gradually as the test progresses, and it exerts a great influence upon the results of the test before the person in charge of the test is aware of it.